The present disclosure relates to automation of three-dimensional (3D) coordinate measurements.
One set of coordinate measurement devices belongs to a class of instruments that measure the three-dimensional (3D) coordinates of a target point by sending a beam of light to the point. The beam of light may impinge directly on the point or on a retroreflector target in contact with the point. In either case, the instrument determines the coordinates of the target point by measuring a distance and two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. The beam may be steered with a gimbaled mechanism, a galvanometer mechanism, or other mechanism.
A tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more beams it emits, which may include light from a laser or non-laser light source. Coordinate-measuring devices closely related to the tracker include the total station. A total station is a 3D measuring device most often used in surveying applications. It may be used to measure the coordinates of a diffusely scattering target or a retroreflective target. Hereinafter, the term tracker (or laser tracker) is used in a broad sense to include trackers as well as total stations and to include dimensional measuring devices that emit laser or non-laser light.
In many cases, a tracker sends a beam of light to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The vertex, which is the common point of intersection of the three mirrors, is located at the center of the sphere. Because of this placement of the cube corner within the sphere, the perpendicular distance from the vertex to any surface of the SMR rests remains constant, even as the SMR is rotated. Consequently, the tracker can measure the 3D coordinates of a surface by following the position of an SMR as it is moved over the surface. Stating this another way, the tracker measures only three degrees of freedom (one radial distance and two angles) to fully characterize the 3D coordinates of a surface.
One type of tracker contains only an interferometer (IFM) without an absolute distance meter (ADM). If an object blocks the path of the beam of light from one of these trackers, the IFM loses its distance reference. The operator must then track the retroreflector to a known location to reset to a reference distance before continuing the measurement. A way around this limitation is to put an ADM in the tracker. The ADM can measure distance in a point-and-shoot manner. Some trackers contain only an ADM without an interferometer.
A gimbal mechanism within the tracker may be used to direct a beam of light from the tracker to the SMR. Part of the light retroreflected by the SMR enters the tracker and passes onto a position detector. A control system within the tracker uses position of the light on the position detector to adjust the rotation angles of the mechanical axes of the tracker to keep the beam of light centered on the SMR. In this way, the tracker is able to follow (track) a moving SMR.
Angle measuring devices such as angular encoders are attached to the mechanical axes of the tracker. The one distance measurement and two angle measurements of the tracker are sufficient to specify a three-dimensional location of the SMR. In addition, several trackers are available or have been proposed for measuring six degrees-of-freedom (six-DOF), rather than the ordinary three degrees-of-freedom. Such six-DOF measuring device may include tactile probes, triangulation scanners, sensors, and projectors, for example.
Many trackers today include one or more cameras. Such cameras may be attached to outer portions of the rotatable tracker frame or may be positioned internal to the tracker. The main uses for such cameras are in determining the location of retroreflectors or in performing six-DOF measurements.
In some cases, a tracker tracks the movement of an SMR and records 3D coordinates of an object when the SMR is brought in contact with the surface of the object. In other cases, a tracker tracks the movement of a six-DOF device and measures 3D coordinates of an object with the six-DOF device. In these types of measurements it is frequently necessary to measure an object from multiple different directions. For example, it may be necessary to measure an auto body-in-white from the front, right side, rear, and back side. When the tracker is moved from location to location to obtain these different 3D measurements, a method is needed for bringing the 3D measurements obtained by the tracker in the different locations into a common frame of reference. A way of doing this in practice is to place at three or more SMRs on kinematic nests spaced around the object to be measured. At each location, the tracker measures the positions of the different SMRs. In each case, the SMR is pointed back at the tracker to allow the beam of light from the tracker to fall within the acceptance angle of the SMR. To do this, an operator walks to each SMR and rotates it to face the tracker. In the case of a fully automated measurement, for example, when a tracker is on a self-propelled mobile stand or the object being measured is on a conveyor belt, it may not be possible for the operator to manually turn each SMR.
Although trackers and other 3D measuring instruments are generally suitable for their intended purpose, the need for improvement remains, particularly in enabling automation of 3D measurements by the tracker or other 3D measuring device.